1. Field of the Invention
This invention relates to an apparatus and control system for heating asphalt to maintain it in a molten state while avoiding the production of volatile air polluting by-products.
2. Description of the Prior Art
Asphalt is widely used as a material for waterproofing roofs because it is relatively inexpensive and in its molten state is relatively easy to apply. Ordinarily, roofing contractors keep a gas-fired kettle at the job site in which solid asphalt is melted and maintained at approximately 500.degree. F. The asphalt when melted in this manner, however, emits a great amount of volatile hydrocarbon vapors into the atmosphere. These hydrocarbon vapors create such air-pollution problems that local governing bodies have proposed regulations restricting the use of the open kettles widely used by roofers for melting asphalt.
Asphalt emits few pollutants when heated to only approximately 500.degree. to 510.degree. F. because it is relatively nonvolatile at that temperature; the asphalt vapor which does escape into the atmosphere from the surface of molten asphalt condenses quickly and therefore is not a serious air-pollution problem. Nevertheless, gaseous pollutants are formed because of the high temperature at which the heating element of the kettle is operated.
Conventional asphalt kettles are heated by gas-fired flame tubes, which are metal tubes immersed in the molten asphalt into which a gas burner directs a flame. Because of the relatively low thermal conductivity of asphalt, the surface of the flame tube must be maintained at 1000.degree. to 1500.degree. F. in order to melt the asphalt within a reasonable time. Although at these high temperatures the bulk of the asphalt melts relatively quickly, the asphalt immediately adjacent to the surface of the flame tube is heated to above its decomposition temperature. The decomposition products of asphalt give rise to air-pollution problems. Since the decomposition products of asphalt include light volatile fractions which can escape into the atmosphere and do not condense at ambient temperatures, they disperse a great distance from the asphalt kettle and constitute a major source of air-pollution from conventional asphalt kettles.
An additional problem arises because the decomposing asphalt deposits coke-like residue on the surface of the flame tube. These deposits are poor heat conductors and thus reduce the efficiency of the heat transfer from the flame tube to the asphalt. Accordingly, the flame tubes of asphalt kettles must be cleaned frequently--a messy and difficult task.
One attempt to solve some of the problems of asphalt kettles heated with flame tubes is a tank truck for delivering molten asphalt. The tank is vented to the atmosphere through a filter which removes condensable vapors generated in the tank. Noncondensable vapors, however, can pass through the filter to the atmosphere. The asphalt in the tank is heated by a flame spreader, which is a large-diameter heating chamber immersed in the tank in contact with the asphalt.
While this development has reduced the air pollutants somewhat by heating the asphalt to lower temperatures than the gas-fired kettles, it nevertheless has many inherent disadvantages. For example, the tank capacity is greatly reduced because of the large volume occupied by the flame spreader. In addition, the temperature of the active surfaces have been so high that coke accumulations--an indicator of asphalt decomposition--have to be periodically removed from the surfaces. Volatile by-products of the coke deposits are not completely prevented from polluting the atmosphere by the relatively simple filter provided. Moreover, as the tank is emptied and the level of asphalt is reduced, the flame spreader becomes exposed with the result that the temperature of the exposed portion increases causing further asphalt decomposition on these surfaces.
Since the physical law which describes the transfer of heat across a surface dictates that the amount of heat carried across the surface is directly proportional to the product of the surface area and the temperature gradient at the surface, it can be seen that increasing the temperature difference between the bulk of the asphalt and a surface in contact therewith increases the rate of flow of heat to the asphalt because the thermal gradient at the surface of contact is increased. However there is a practical limit to the rate of heat which can be achieved by this method since as the temperature of the surface approaches the decomposition temperature of asphalt, serious air pollution problems result. Accordingly it follows from the law of heat transfer that for a fixed flow of heat to the asphalt, the thermal gradient can be reduced if the area in contact with the asphalt is increased proportionately. I have invented an apparatus and a control system for heating asphalt to temperature levels sufficient to maintain it in a molten state by increasing the area in thermal contact with the asphalt and thereby reducing the thermal gradient thus preventing decomposition of the asphalt, minimizing the formation of atmospheric pollutants, and avoiding the disadvantages of the prior art.